FIG. 15 is a flow diagram of a conventional air-conditioning system. As shown in FIG. 15, there has heretofore been available a dehumidifying air-conditioning apparatus having a compressor 201 for compressing a refrigerant, a condenser 202 for condensing the compressed refrigerant with outside air OA, an evaporator 204 for depressurizing the condensed refrigerant with an expansion valve 203 and evaporating the refrigerant to cool process air from an air-conditioned space 100 to a temperature lower than its dew point, and a reheater 205 for reheating the process air, which has been cooled to a temperature lower than its dew point, at the downstream side of the condenser 202 with the refrigerant upstream of the expansion valve 203. With the illustrated dehumidifying air-conditioning apparatus, a heat pump HP is constituted by the compressor 201, the condenser 202, the reheater 205, the expansion valve 203, and the evaporator 204. The heat pump HP pumps heat from the process air which flows through the evaporator 204 into the outside air OA which flows through the condenser 202.
FIG. 16 is a Mollier diagram in the case where HFC134a is used as the refrigerant in the conventional dehumidifying air-conditioning apparatus. In FIG. 16, a point “a” represents a state of the refrigerant evaporated by the evaporator 204, and the refrigerant is in the form of a saturated vapor. The refrigerant has a pressure of 0.34 MPa, a temperature of 5° C., and an enthalpy of 400.9 kJ/kg. A point b represents a state of the vapor drawn and compressed by the compressor 201, i.e., a state at the outlet port of the compressor 201. In the point b, the refrigerant is in the form of a superheated vapor.
The refrigerant vapor in the state represented by the point b is cooled in the condenser 202 and reaches a state represented by a point c in the Mollier diagram. In the point c, the refrigerant is in the form of a saturated vapor and has a pressure of 0.94 MPa and a temperature of 38° C. Under this pressure, the refrigerant is cooled and condensed to reach a state represented by a point d. In the point d, the refrigerant is in the form of a saturated liquid and has the same pressure and temperature as those in the point c. The saturated liquid has an enthalpy of 250.5 kJ/kg.
The refrigerant liquid is depressurized by the expansion valve 203 to a saturation pressure of 0.34 MPa at a temperature of 5° C. and reaches a state represented by the point e. The refrigerant at the point e is delivered as a mixture of the refrigerant liquid and the vapor at a temperature of 5° C. to the evaporator 204, in which the mixture removes heat from process air and is evaporated to reach a state of the saturated vapor, which is represented by the point a in the Mollier diagram. The saturated vapor is drawn into the compressor 201 again, and the above cycle is repeated.
FIG. 17 is a psychrometric chart showing an air-conditioning cycle in the conventional dehumidifying air-conditioning apparatus. In FIG. 17, the alphabetical letters K, L, M correspond to states in paths indicated by the encircled letters in FIG. 15. As shown in FIG. 17, in the conventional dehumidifying air-conditioning apparatus, air (in a state K) from the air-conditioned space 100 is cooled to a temperature lower than its dew point to lower the dry bulb temperature thereof and lower the absolute humidity thereof, and reaches a state L. The state L is on a saturation curve in the psychrometric chart. The air in the state L is reheated by the reheater 205 to increase the dry bulb temperature thereof and keep the absolute humidity thereof constant, and reaches a state M. Then, the air is supplied to the air-conditioned space 100. The state M is lower in both of absolute humidity and dry bulb temperature than the state K.
With the conventional dehumidifying air-conditioning apparatus described above, since it is necessary to considerably cool the air to its dew point, about 50% of the cooling effect of the evaporator in the heat pump is consumed to remove a sensible heat load from the air, so that the moisture removal (the dehumidifying performance) per electric power consumption is low. If a single-stage compressor is used as the compressor in the heat pump, then it produces a one-stage compression-type refrigerating cycle, resulting in a low coefficient of performance (COP) and a large amount of electric power consumed per amount of moisture removal.